(1) Field of the Invention
The invention relates to the manufacture of highly dense integrated circuits, and more particularly to the formation of field oxide isolation regions by selective oxygen implantation within the integrated circuits.
(2) Description of the Related Art
In the manufacture of highly dense integrated circuits, individual device structures are typically separated and electrically isolated by means of a field oxide isolation region. The isolation region is typically produced by the exposure of a silicon wafer to an oxidizing atmosphere while using an oxidation mask to protect regions which are not desired to be oxidized. These latter regions will be the location for the active device structures. One widely used technique for creating isolation regions is LOCOS-LOCal Oxidation of Silicon.
In the LOCOS technique, a pad oxide is grown on the surface of a silicon substrate. The pad oxide reduces stresses during field oxidation between the silicon substrate and a subsequently deposited silicon nitride layer. The silicon nitride is deposited by chemical vapor deposition (CVD) on the surface of the pad oxide, and then patterned to create an oxidation mask. The silicon nitride forms an effective mask during field oxidation due to the slow speed with which oxygen and water vapor diffuses in the nitride. The oxidation mask is formed by dry etching the nitride and pad oxide in the region in which it is desired to form the field oxide. An implant is performed in the field region to create a channel-stop doping layer under the field oxide. The field oxide is then grown by wet oxidation, at a temperature of about 1000.degree. C. The masking layer is removed by a wet etch. A sacrificial pad oxide is then formed and removed, to prepare for subsequent formation of the gate oxide.
The LOCOS technology has been used widely, but it presents several drawbacks at dimensions between 1 and 2 micrometers, and for submicron technology the conventional LOCOS cannot be used without significant modification. Attempts have been made to provide field isolation for submicron devices, but many provide only partial solutions. Particularly, the problem of punchthrough (leakage and isolation issues) have not been adequately solved. If the silicon surface under the field oxide is inverted, the two transistors may short and isolation between them is effectively broken down. When the spacing between active devices, and the field oxide thickness, are scaled down as is necessary for CMOS (Complementary Metal Oxide Semiconductor) VLSI (Very Large Scale Integration), leakage current paths are more likely to form, causing unwanted DC power dissipation, noise margin degradation, and voltage shift on a dynamic node.
Other problems which have not been adequately resolved include "bird's beak" formation and lack of latch-up immunity. During oxidation, there is significant lateral oxidation encroachment from under the nitride mask in conventional LOCOS, resulting in the formation of a "bird's beak" structure at the perimeter of the field oxide, which reduces the area available in the active region (also known as the narrow width effect), thus lowering the total number of devices that can be fit onto a single integrated circuit chip. Furthermore, as CMOS feature sizes continue to shrink, lateral and vertical dimensions are scaled down, resulting in better parasitic bipolar transistors. Leakage currents under the field oxide can cause forward biasing emitter-base junctions, activating both bipolar devices. Latch-up, which occurs in CMOS devices, can result in momentary or permanent loss of circuit function.
Workers in the field are aware of these problems. One approach is to use a shorter oxidation period as can be obtained by implantation of oxygen in the field region, prior to field oxidation, as described in "Selectively Implanted Oxygen Isolation Technology (SIO)", P. Ratnam et al, Electronics Letters, May 9, 1985, Vol. 21 No. 10, pp. 442-443. Oxygen is selectively implanted to create a damaged Si--SiOx--Si layer in the field region, which is then thermally oxidized for a short period to form the field oxide.